The session deserializer in PHP before 5.4.45, 5.5.x before 5.5.29, and 5.6.x before 5.6.13 mishandles multiple php_var_unserialize calls, which allow remote attackers to execute arbitrary code or cause a denial of service (use-after-free) via crafted session content.
http://php.net/ChangeLog-5.php
https://bugs.php.net/bug.php?id=70219
https://security.gentoo.org/glsa/201606-10